1. Field of the Invention
The present invention generally relates to lenses for devices such as digital cameras and, more particularly, to a lens that has lens elements with a diffractive surface.
2. Discussion of the Related Art
Digital cameras utilizing high-resolution electronic imaging sensors typically require high resolution optical components such as lenses. In addition, the lenses generally must be very compact, so that they can be incorporated into devices such as palm-sized computers, cellular telephones, and the like.
Lenses for digital cameras generally have several individual lens elements. The lens elements are typically spherical and usually create spherical aberration. Chromatic aberration, coma aberration, distortion, and field curvature are also common optical aberrations that occur in the imaging process of a typical lens (http://www.astrosurf.org/lombry/report-aberrations2.htm). A large number of lens elements are generally required in order to balance the inherent optical aberrations. Lenses having a large number of lens elements tend to be large, heavy, and expensive to manufacture. The manufacturing costs involve both significant material costs and the cost of assembling and mounting the lens elements into a lens cell.
Further, conventional lenses commonly use one or more aspheric lens elements, each of which has one or two non-spherical surfaces. The aspheric lens elements are made of plastic or glass. Aspheric plastics lens elements may be produced by means of plastic injection molding and are therefore relatively inexpensive. However, the optical characteristics of most plastic lens elements change with changes in temperature and humidity, such as when the digital camera is used outdoors on very hot or very cold days. Furthermore, the hardness of optical plastic material is lower than that of an optical glass material. The surfaces of such lens elements are easily scraped or abraded, which affects the precision of the imaging. In comparison, glass aspheric lens elements have good optical properties and are less easily scraped or abraded. However, glass aspheric lenses generally cannot be easily produced by traditional glass grinding and polishing techniques. In addition, glass lens elements are heavier than plastic lens elements and thus go against the trend toward lightweight digital cameras.
A typical lens having both spherical lens elements and aspheric lens elements is disclosed in China Patent Number 02258384. The lens includes a first spherical lens element, a second spherical lens element, and a third lens element. The first lens element and the second lens element are made of glass. The third lens element has two aspheric surfaces and is made of plastic. Although the lens may satisfy the requirements for imaging, the resolution of the lens is low and may affect the image performance.
Accordingly, what is needed is a lens system for a digital camera which is compact and which provides good imaging quality.